Optical fiber processing method

ABSTRACT

The invention relates to a process of preparing an optical fiber joint, wherein an optical fiber or its undrawn precursor is brought into contact with a longitudinal element and attached to it via a ridge-groove fitting prior to drawing of the final optical fiber joint. The longitudinal element can be another optical fiber or its precursor or a removable and/or workable bridging element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process of preparing an optical fiberjoint and to a process for preparing a bundle containing optical fibersapplying the said fiber joint process. Bundles containing opticaldevices are useful in fiber lasers and amplifiers, in particular whencoupling light from multimode pump sources to one or more doublecladding fibers including an active element in the fiber core.

2. Description of the Related Art

The output powers from double-clad fiber lasers have recently increasedtremendously. In recent experiments, hundreds of watts have beenachieved from essentially single-mode devices making fiber lasersserious contenders for e.g. lamp and diode pumped (Nd:Glass) Nd:YAG inmaterial processing. Apart from continuous wave operation, pulsedoperation and pulse amplification with down to femtosecond range pulsesand peak powers well in excess of 100 kW have also been demonstratedwith fiber lasers.

Fiber lasers based on double-clad fibers offer several benefits oversolid state lasers. The main performance advantage of the double-cladfiber lasers is the easier management of the thermal load within thedevice. The high surface to active volume ratio of such fibers ensuresexcellent heat dissipation. Also, the beam modal quality is largelydetermined by the size and refractive index profile of the active coreand the surrounding materials, and is therefore independent of the pumppower and the thermal load. In comparison, solid-state lasers employingbulk crystals as their gain media are known to be sensitive to sucheffects as thermal lensing, which have to be taken into account whendesigning the laser cavity. In this respect, the fibers provide aremarkably stable gain element. In addition, further benefits of fiberlaser such as spatial compactness, highs degree of integration,simplicity of operation, robustness and reliability of operation aredriving the rapid adaptation of high power fiber lasers and fiberamplifiers into several commercial and military applications, forexample, as sources for micro-machining, thermal printing, welding,marking, medical applications, and remote sensing.

Traditional double clad active fibers consist of a rare earth dopedsingle-mode or multimode core, a silica inner cladding with a diameterof several hundred micrometers and a second low index outer claddingconsisting of a low index polymer such as silicone resin. The numericalaperture (NA) between the inner and outer cladding is in the range0.35-0.47 depending on the refractive index of the coating material. Theinner cladding is used to couple pump radiation from low-brightnessmultimode pump sources such as high power diode lasers, diode laser barsand arrays for absorption in the active core. Typically, the overlapbetween the pump field and the doped core is small which requires longdevice lengths to absorb the majority of the pump radiation. Pumpabsorption in the core material can be enhanced by fiber structuredesign where the single-mode core is offset from the centre of thecircular cladding or where the circular symmetry of the inner claddingis broken.

A common problem related to double clad fiber devices is how to couple asufficient number of low brightness sources to the inner claddingefficiently and without significantly increasing the cost of the fiberlaser. The pump coupling problem is enhanced by the requirement ofsimultaneous coupling of the signal light into and out of the activecore. Being able to separately couple pump light into the double cladfiber structure without affecting the signal coupling allows the use ofbidirectionally pumped cladding-pumped fiber lasers and fiberamplifiers. The output power scaling of the bidirectionally pumped fiberlaser and amplifiers is more straightforward as multiple pumping pointscan be added along the fiber length. Additionally, the thermalmanagement of the bidirectionally pumped structures is simplified.However, the problem of careful and stable signal coupling into and outof the double clad fibers has become more important because of theadaptation of large-mode area (LMA) or multimode fibers in high power CWfiber lasers and short pulse fiber amplifiers. LMA fibers are requiredto overcome the output power (peak power for short pulse applications)limitations imposed by fiber nonlinearities. For these fibers theexcitation of the fundamental mode is essential to achieve close tosingle-mode operation (see e.g. U.S. Pat. No. 5,818,630).

Fiber structures have been developed, which enable the coupling of pumplight into the signal fiber. Most of these structures are optimized tooperate at lower power levels having a single-mode core. In order toboost the output powers in the kW level, the core size of the signalfiber needs to be increased. The increase in core size is required toovercome the thermal limitations in the fiber core and to the increasethe threshold for nonlinearities such as stimulated Raman scattering,stimulated Brillouin scattering and self-phase modulation. In high powerfiber laser having diffraction limited output beam quality, thereduction of the microbending of the optical fiber is essential toeliminate the mode-mixing in the signal core. Also the number ofsplicing points need to be minimized as they are also possible pointsfor mode mixing and they increase the background losses of the fiber(thus reducing the efficiency of the fiber laser).

Photonic Fibers

Recently, “photonic fibers” have appeared on the market, these fibersalso being known as “photonic crystal fibers” (PCF) (see U.S. Pat. No.6,778,562). Like conventional fibers, these fibers are not fullyconstituted by a solid transparent material, such as doped silicon.Shown as a section, a photonic fiber exhibits a plurality of air holesof another gas, even a vacuum. The only known function of the holes isthe effect of inducing in the fiber large variations of the index, thesevariations, like the variations induced in the fiber by doping agents,contributing to guide the light in the fiber (see U.S. Pat. No.6,778,562, FR2822242, CA2362992, WO00/49435, US2004/071423,US2004/0052484 and CA2368778).

The holes of these photonic fibers are parallel to the axis of the fiberand extend longitudinally along the fiber. In practice, these holes canbe obtained by producing a preform by assembling silicon cylinders orcapillary tubes according to the required pattern of the holes in thedrawn fiber. Finally, stretching this preform provides a fiber withholes corresponding to the pattern of capillary tubes. Said U.S. Pat.No. 6,778,562 discloses the bundling and stretching of photonic fiberstogether with conventional optical fibers to give a bundle structurehaving photonic fibers surrounded by optical fibers. The stretchingusually closes the openings of the photonic fibers.

Optical Couplers

One common approach to couple the light into the fiber is to use bulkoptics to couple the light directly from the low brightness source(s) orfrom a multimode fiber(s) into the double clad fiber. In order to haveboth ends of the double clad fiber free for signal coupling light can becoupled into the fiber by using a bulk prism (see U.S. Pat. No.4,815,079) or a V-groove which is fabricated into the side of the fiber(see U.S. Pat. No. 5,854,865 or L. Goldberg and J. Koplow, High powerside-pumped Er/Yb doped fiber amplifier, Technical Digest of the OpticalFiber Communication Conference (OFC), 2, 19-21, 1999/) as is shown inFIG. 1( a). The difficulty in these approaches is matching and alignmentof different components well enough to get acceptable couplingefficiencies. Bulk components can be avoided by angle polishing the endof the multimode fiber and attaching the fiber to the side of thedouble-clad fiber for example by soldering, UV-curing or epoxyfying(U.S. Pat. No. 6,370,297) as is shown in FIG. 1 (b). Different surfacesand components, however, require also polishing, antireflection coatingand maintenance of the good alignment, which further complicate themanufacturing and increase the cost of these systems.

WO96/20519, U.S. Pat. No. 5,999,673, “A coupling arrangement between amulti-mode light source and an optical fiber through an intermediateoptical fiber length” is a similar pumping arrangement to U.S. Pat. No.6,370,297, but here the pump light is introduced to the pump claddingthrough an intermediate fiber length which is fused and tapered to thesignal fiber as shown in FIG. 2. This reduces the probability of thethermal damage but does not solve the manufacturing difficulties.

Another approach is to use separate components such as fused fiberbundles (U.S. Pat. Nos. 4,291,940, 5,864,644, 6,397,636, 6,434,302 and6,778,562) or fused fiber tapers (U.S. Pat. No. 599,673), which arespliced into the double-clad fibers as shown in FIG. 3. These solutionsavoid the use of any bulk optic components in the pump coupling and alsodo not require any antireflection coatings on the coupling surfaces asthey are all-fused solutions. In these structures several multimodefibers are typically bundled together, heated and stretched to form afused fiber bundle having a diameter smaller than the combined diameterof the fiber bundle prior to heating and stretching process. Thedifficulty in achieving high transmission of multimode light through thefiber bundle is to keep the fiber deformations low during the heatingand stretching of the bundle while fusing the fibers together to form abundle which matches the diameter of the double clad fiber. This shouldbe done by fusing the fibers together so that any gaps ordiscontinuities in between the fused fibers are avoided. A common methodto achieve this is to apply tensile force to the twisted or pulledportion of the bundle after which the bundle is heated and stretched sothat the fibers melt together (U.S. Pat. Nos. 4,291,940, 5,864,644 and6,434,302). However, using these approaches the making of the high powerfused bundles tolerating output powers in excess of hundreds of watts isdifficult because even a few micrometer variations in the diameter ofthe circular fibers being fused together causes the formation of a gapin between the fibers. These methods have been improved by the use of aprecursor material (U.S. Pat. No. 6,397,636) or manufacturing a specialphotonic fiber (U.S. Pat. No. 6,778,562) to minimize the appearance ofthe gaps in between the fiber. Both of these approaches may reduce therisk of having any gaps or discontinuities in between the fused fibersbut one is still left with the problem of careful control of the signalfiber core size variations in the melting and above all stretching ofthe bundle. These core size variations induce losses to signal and canalso excite higher order modes therefore degrading the beam quality ofthe fiber laser or fiber amplifier.

Another all-fiber coupling architecture introduced in U.S. Pat. No.6,434,295, which describes a fiber bundle architecture, wherein multiplemultimode pumps are coupled into a single multimode-, pump fiber. Thispump fiber is further sandwiched in between two double-clad fibers twoform a fused coupler section in which all the fibers share a commoninner-cladding and two-thirds of the pump light is coupled to thedouble-clad fiber. The coupled-in pump light travels further to a fiberloop having a typical length corresponding to 10 dB pump absorption. Theresidual pump light in the pump fiber travels to a second couplingmodule where another two-thirds of the pump power is absorbed. Theapproach in this invention is similar to the use of fused fiber bundlein that separate coupler modules are used to couple the pump light intothe double-clad fiber. The amplifier fibers are arranged around the pumpfiber and heating as well as fiber pulling parameters to form the fusedcoupling are chosen to ensure good melting among the fiber innercladding and the silica fiber. Unfortunately, the manufacturing of thecouplers is still done separately and is extremely sensitive to anyperturbation in the fusing process, which may excite higher order modesand cause extra signal and pump losses. Therefore the use of suchcouplers in high power fiber lasers/amplifier based on large-mode areafibers creates additional difficulties in maintaining the beam qualityclose to single-mode behavior.

SUMMARY OF THE INVENTION

The manufacture of the above prior art optical devices such as fiberlasers, couplers and amplifiers require in themselves tapering andstretching measures which cause variations in the cross section of thefibers, especially in the cross sections of the core of single anddouble cladding fibers. This leads to a degradation of the beam qualityproduced by the devices. Further, the devices require severalmanufacturing steps, which make their manufacture laborious andexpensive. Also, the devices are difficult to couple with each other andlight sources such as laser diodes, especially if many diodes are used(multiple pumping points are installed along the fiber length).

It is an object of this invention to provide a fused all-fiber fusedapproach for coupling the multimode pump light into the fiber. Such anarrangement is achieved without the tapering and stretching processesthat would induce core size variations and, degrade the beam quality ofthe fiber lasers and amplifiers.

Another object of the invention is to integrate the functionality forinjecting the pump light into the double-clad fiber in the fibermanufacturing process in order to minimize any core size variations inthe structure and to avoid the manufacturing of separate pump couplers.

A further object of this invention is to allow the pumping of the fibergain media with multiple discrete fiber-coupled laser diodes in adistributed pumping architecture (multiple pumping points along thefiber length).

The above mentioned problems have now been solved by a bundle containingoptical fibers, which is essentially characterized in that at least twoof the optical fibers are attached to each other lengthwise by means ofat least one longitudinal element. This longitudinal element can be anoptical fiber or its undrawn precursor or a bridging element, which isworkable and/or removable in order to detach the two fibers from eachother. The fibers are joint in a process, wherein an optical fiber orits undrawn precursor is brought into contact with a longitudinalelement and attached to it via a ridge-groove fitting prior drawing ofthe final fiber joint. The resulting new type of microstructured fiberhas many advantages.

By optical fiber bundle is here meant any fiber bundle, which is able toreceive an optical signal, amplifying it, and/or modifying it and/ortransmitting it further. By optical fiber is meant both a normal fiber,which transmits an optical signal as well as a precursor orsemi-manufactured product thereof. Thus, in addition to conventionallymeant fiber bundles, also bundles of glass rods and tubes may accordingto the claimed invention be provided with a workable and/or removablebridging element and optionally subjected to its at least partialremoval followed by stretching into the conventionally meant fiberbundle. By “attached to each other lengthwise” is meant that the opticalfibers are attached to each other along their whole length or only alonga section of their whole length.

By “workable and/or removable” is meant, firstly, that the material ofthe bridging element is workable and/or removable by a working method.Secondly, the term means that the bridging element is so built, arrangedand/or of such material that it is available to a working and/orremoving action. This can e.g. mean that there are slits formed betweenthe surrounding fibers through which the bridging element can be workedexternally. It can also mean that there is a hole in the bridgingelement through which it can be worked internally. Naturally acombination of said internal and external availability can be the case.Examples of bridging elements are described in the detailed descriptionof the invention below. By working is meant any action, which isdirected to, and changes the bridging element or any material unitingthe bridging element and the optical fibers. In practice, it may meanmachining by physical means such as a laser or by mechanical means suchas a blade made of hard material. It may also mean working by chemicalmeans such as etching, leaching or dissolution.

By removing one or more sections of the removable longitudinal bridgingelement, the optical fibers are in those sections freed from each otherand can be used for optical coupling, which may take place at the endand/or in the middle of the fiber bundle. Thus, the instant inventionallows the pumping of a fiber laser from a single point with pump powersexceeding 1 kW, as well as dividing this pump power into multiple pointsand distributing it along the whole fiber laser length without havingany additional signal splices along the way. In the structure, thefibers are attached to each other by the bridging element to form asolid all-fused fiber structure.

In the present invention, both the optical fiber and its coupling meansare manufactured together in a highly simplified and economic way. Thisintegrated manufacturing of e.g. an active fiber and its pump coupler isabout 20% cheaper than separate manufacturing. Furthermore, unwantedvariations in the mechanical and optical properties of such a productcan be avoided.

BRIEF DESCRIPTION OF THE DRAWING FIGS.

In the following, the invention is described more closely by examplesand the following figures in which:

FIG. 1 (Prior Art) (a) describes V-groove side pumping, (b) describesangle polished side coupling.

FIG. 2 (Prior Art) describes a coupling arrangement between a multi-modelight source and an optical fiber through an intermediate optical fiberlength.

FIG. 3 (Prior Art) describes a fused multimode coupler incorporating asignal fiber in the center of the fused bundle. Coupler is furtherspliced to an active double clad fiber.

FIG. 4 describes the joining by two bridging elements according to theinvention.

FIG. 4 a) describes the preforms before joining utilizing capillarytubes, b) all-glass joints as bridging elements, and c) a sacrificialcladding layer on a lower optical fiber as a bridging element.

FIG. 5 describes a clover-shaped embodiment of the claimed bundle.

FIG. 6 describes a keyhole-shaped embodiment of the claimed bundle.

FIG. 7 describes a double keyhole-shaped embodiment of the bundle.

FIG. 8 describes a linear-aligned and compact embodiment of the bundle.

FIG. 9 describes another compact embodiment of the bundle.

FIG. 10 describes a clover-shaped embodiment of the claimed bundle wherethe bridging element is a sacrificial cladding layer on an opticalfiber.

FIG. 11 describes a fiber laser or fiber amplifier bundle of theinvention.

FIG. 12 describes another fiber laser or fiber amplifier bundle of theinvention.

FIG. 13 describes a pump coupler according to the invention.

FIG. 14 describes a further linear-aligned fiber laser or fiberamplifier of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The optical fiber bundles are manufactured via a specific process toprepare an optical fiber joint. In this, an optical fiber or its undrawnprecursor is brought into contact with a longitudinal element andattached to it in a manner, where the first object selected from saidoptical fiber or its precursor and said longitudinal element is providedwith a ridge and second object selected from said optical fiber or itsprecursor and said longitudinal element is provided with a groove, theridge is fitted into the groove, the optical fiber or its precursor andthe longitudinal element are attached to each other, and, in the case ofthe optical fiber precursor, the attached optical fiber and the opticalfiber precursor are drawn, to give the said optical fiber joint. Thelongitudinal element can be an optical fiber or its undrawn precursor ora bridging element, which is workable and/or removable in order todetach the two fibers from each other.

The said longitudinal elements can be provided with several parallelalternating ridges and grooves, the ridges and the grooves of saidlongitudinal elements being engaged with each other. Preferably, theparallel alternating ridges and grooves form two saw-alike patterns,which are engaged. The optical fiber or its precursor and thelongitudinal element are attached to each other by fusing.

Then, when removing (etching, melting, cutting, etc.) the bridgingelement, the loosening of the optical fibers for further couplingbecomes easy. It is advantageous if the fibers are attached at adistance from each other exposing the bridging element to said working.This is especially the case when the working of the bridging element inorder to release the fibers is carried out from the outside (in contrastto internal working eg. by etching through a hole in the bridgingelement).

According to an embodiment of the invention, the bridging elementconsists of a material, which has essentially the same refractive indexas the material of the outer layer of the fibers. Preferably, thebridging element is made of the same material as the outer layer of thefibers. This means that the bridging element not only functions asattaching means between the fibers, but also is optically active andhelps to guide the light. Usually, it helps to spread the light, therebye.g. increasing the mode mixing of pump light and enhancing theabsorption of pump light into the active fiber. Typically, the materialof the bridging element is a light transmitting material such as silicaglass or material, which is more easily workable and/or removable thanthe material of the optical fibers. Preferably, the material is silicaglass doped with a material, such as phosphorous, the silica glass ofthe bridging element being doped thus to make it workable/removable. Inorder to ensure unhindered light transmission through the bridgingelement, it is advantageously attached to at least one of the fibers byfusing. Then, no boundary layer prevents the pump radiation fromspreading through the bridging element and being absorbed into thelaser.

The bridging element is according to the invention so constructed andattached that it is workable and/or removable in a technicallyacceptable manner. It can be attached to the fibers in a normal way byfusing. In order to attach the fibers and the bridging element in anordered manner, which allows working of the bridging element, attachmentby male-female coupling is advantageous. Most preferably, the couplingtakes place by engaging a number of alternating ridges and grooves inthe bridging element and fibers in question. The ridges and grooves maybe accomplished e.g. by sawing. After male-female coupling, the bridgingelement and the fibers may advantageously be fused together in order toaccomplish an optically homogeneous joint.

In one embodiment of the invention, the bridging element is a separateelement to which said at least two fibers of the bundle are attached.Preferably, it is a longitudinal element having a cavity or hole, whichcan be subjected to internal working through said capillary. Such anelement has two advantages. Firstly, the cavity or hole of a not removedportion thereof may be filled with a functional substance, whichmodifies the properties, especially the optical properties, of thebundle. One such substance is an optical stress inducing material.Secondly, in order to move at least a portion thereof, an etchingsubstance may be sucked or pressed into the cavity or hole, which etchesaway the portion and releases the fibers. For the last mentionedpurpose, it is advantageous to have thin walls. Therefore, the wall ofthe bridging element having a cavity or hole has a smallercross-sectional area than the fibers. Most preferably, the bridgingelement is a one-hole capillary. The capillary differs from prior artmulticapillary photonic fibers, the capillaries of which are mostlyclosed and with differing inner diameters due to fiber drawing methodsemployed, and/or so thin that an introduction of functional substancesis technically impossible. Preferably, the diameter of the hole is from20 to 200 μm.

According to another embodiment of the invention, the longitudinalbridging element is a projection protruding from and running along oneof the fibers and being attached to the other of the fibers. When such aprojection is attached to another fiber by fusing, it unites the fibers.The projection can then be removed e.g. by sawing or laser welding, seebelow, thereby releasing the fibers from each other. Preferably, theprojection engages the fiber along which it runs to another fiber bysaid male-female coupling. Most preferably, the engagement takes placeby male-female coupling followed by fusion.

According to one embodiment of the invention, the bridging element is asacrificial layer, which typically surrounds the optical fiber. Thiscladding material is preferably a silica glass doped with a material,such as phosphorous, that increases the wet etching speed. The opticalfibers are joined together via this sacrificial glass layer. Fibers areattached to said bridging element in such a manner that there are slitsin the coating material formed between the surrounding fibers throughwhich the sacrificial layer can be worked externally. The sacrificiallayer can then be removed by wet or dry etching, thereby releasing thefibers from each other. Preferably, the sacrificial layer engages thefiber along which it runs to another fiber by said male-female coupling.Most preferably, the engagement takes place by male-female couplingfollowed by fusion.

In the invention, the number of bridging elements can vary. Althoughemphasis has been laid on constructions involving only one bridgingelement holding together many optical fibers, see below, the scope ofprotection also extends to bundles having more than one bridgingelement. See e.g. FIG. 7, which shows the use of two bridging elements.

It is preferable if most of the bundle length comprises an outercladding surrounding the fibers and bridging element comprising saidlight transmitting material. The cladding most preferably consists of amaterial having a lower refractive index than the light transmittingmaterial.

In one alternative of the invention, the fibers and the bridging elementare arranged in a geometry allowing bending only in a limited number ofdirections defined by the geometry. Preferably the fibers of the bundleare arranged in a geometry having a significantly larger width in onedirection than in other directions, thereby allowing bending only aboutthe axis of said one direction.

The optical fibers of the claimed bundle can be of any type and number,depending on the intended application. Preferably, the optical fibersare selected from single cladding fibers with a core of higherrefractive index than said light transmitting material and a cladding ofsaid light transmitting material, and multimode fibers consistingessentially of said light transmitting material. Typically, the singlecladding fibers are signal fibers or their precursors and the multimodefibers are pump fibers or their precursors. By precursor is here meantthat they may be undrawn rods and/or longitudinal elements which onlyupon coating with the above-mentioned outer cladding and/or drawing formworking signal or pump fiber structures. However, signal fibers may alsohave multiple cladding layers if that is desired for example to improvethe fiber properties. Signal fibers also have a core (13) where thesignal light travels. The core 13 of the fiber 12 can be doped withactive atoms, such as erbium (Er), neodymium (Nd) or ytterbium (Yb),thulium (Th) or other rare earth atoms to provide gain in the laser oramplifier by optical excitation or pumping of those atoms. Typically,pump fibers don't have cladding layers. However, even in pump fibers,the use of cladding layers in order to improve the properties of fiberstructure is in some cases beneficial.

The bundle in the device according to the invention typically has astructure in which one of the single cladding fibers and one of themultimode fibers are adjacently attached to each other via the bridgingelement. According to one main embodiment, the single cladding fiber,the bridging element and the multimode fiber are, in said order,arranged in a parallel-aligned conformation and surrounded by the outercladding. Then, the elements may have different cross-sectional areas,e.g. the multimode fiber may have a larger cross-sectional area than thesingle cladding fiber and the bridging element, forming a so calledkeyhole profile. A further structure is obtained if one of the singlecladding fibers is parallel-aligned with, in outwards order, twobridging elements and two of the multimode fibers, and surrounded by theouter cladding (see the examples below). One bridging element may beattached to three of the multimode fibers and one of the single claddingfibers and surrounded by the outer cladding. This construction may bearranged into a structure having a four clover-formed cross-section.

One important advantage when using the bridging element according to theinvention is that it can easily be removed to detach the optical fibers.Thus, the claimed bundle may have a section consisting of said at leasttwo of the optical fibers which are detached from each other. Thedetachment is carried out by at least partly working and/or removing thecorresponding length portion of the bridging element between the fibers.Typically, said portion of the bridging element has been removed by amethod selected from wet etching, dry etching, CO₂, UV and ultra fastlaser micromachining as well as ion milling or a combination of saidmethods.

If the bundle raw material has an outer cladding surrounding the fibersand the bridging element, it is removed prior to the removal of thebridging element, e.g. by cutting and/or melting.

In one embodiment of the invention, the bundle portion constitutes anend section having loose optical fibers. Typically, after detaching theoptical fibers from each other at the end section of the bundle, thefibers are (again) individually covered with essentially the samematerial as that of said outer cladding. If at least one bundle portionis tapered, the tapered bundle portion constituted a tapered endportion. If the other end of the bundle is tapered, the bundle may forma typical coupler device. In a further embodiment of the invention, thefiber bundle consists of a bundle section of which at least two of theoptical fibers are detached from each other. In this kind of bundleintermediate section at least one of the fibers is preferably cut inorder to establish optical connection. The said bundle section can bethe other end section of the bundle. If the other end of the bundleconsists of optical fibers, which are also detached from each other, thebundle may form a typical fiber amplifier device.

This invention relates to a process for the preparation of an opticalfiber joint resulting in an optical structure. Further, the inventionrelates to a process for the preparation of a bundle containing opticalfibers. In the invention, a bundle is prepared by attaching at least twoof its optical fibers to each other lengthwise by means of at least onelongitudinal element. This longitudinal element can be an optical fiberor its undrawn precursor or a bridging element, which is workable and/orremovable in order to detach the two fibers from each other. The detailsof this process are disclosed above in connection to the bundle andbelow in claims 1 to 48.

The main advantages of the invention are as follows:

-   -   Provides mechanically reliable and stable coupling from the        laser waveguide to pump waveguide when compared to end pumping        schemes    -   The fiber length can be defined after drawing    -   The pump fiber geometry and the active fiber geometry can be        chosen independently    -   The fiber geometries allowing the bending of the final fiber        only in a limited number of directions are accomplished    -   Excellent and economically efficient optical coupling from pump        waveguide to signal waveguide    -   Improved M²-value and beam parameter product    -   Efficient transfer of energy from the pump light in to the doped        core due to anti-circular geometry of the pump cladding which        force the light rays to interface with the core    -   Less operational steps required when manufacturing laser devices    -   Freedom to operate with several pumping schemes

This invention allows the pumping of the fiber laser from single pointwith the pump powers in excess of kW as well as dividing the pump powerinto multiple points and distributing the pump power along the wholefiber laser length without having any additional signal splices alongthe way. In the structure the signal fiber(s) and pump fiber(s) arefused together via a bridging element (typically capillary tube) to forma solid all-fused inner cladding structure where the pump lightpropagates freely between the signal and pump fiber and provides pumplight for the active core element. Fibers can be detached from eachother by removing the capillary tube from between them for example byetching or by laser micromachining (CO₂, excimer or ultrafast lasers) aswell as ion milling. All the fibers can then be processed by standardfusion splicing techniques to splice multimode fibers or fused multimodefiber bundles to the pump fibers. Also the signal fiber having a dopedcore can be spliced to input and output signal fiber. In the detachmentprocess the doped core does not experience any deformation of the corediameter and hence the splice between the input and output signal fibercan be matched well without the excitation of the undesirable higherorder modes.

The purpose of the bridging elements in the structure is three-folded.Firstly to fuse the active fiber elements and pump fiber(s) together toform a solid all-fused inner cladding structure where the pump lightpropagates freely between the signal and pump fiber and provides pumplight for the active core element as shown in FIGS. 5, 6, and 7.Secondly the bridging element acts as a separating element for injectingpump light as it can locally removed for example by etching or by lasercutting leaving the fiber from the composite fiber end free for splicingsignal and pump fibers. Thirdly, it brings additional functionality tothe double-clad device. For example in case of a capillary tube actingas a bridging element the air-hole in the middle of the capillary tubeimproves the mode mixing of the pump light within the structuretherefore enhancing the absorption of the pump light in the activefiber. Additionally, the air-hole in the structure can be filled with asuitable material for example to increase the birefringence of thefiber.

The number of each element in the structure can vary depending on thedifferent needs required by the applications. Structure can consist ofone or more pump fibers as well as multiple signal fibers. Without theactive core element the fused bundle acts as a pure pump coupler. Alsothe number of capillary tubes fusing the active and pumping elementstogether can vary. The fusing of the fibers can also be done without theuse of capillary tubes as shown in FIGS. 8, 9 and 10. In the cases ofFIGS. 8 and 9, however, separate detachment points (perpendiculargrooves on the fiber surface locating along the fiber length) have to bedone prior to the drawing of the microstructured fiber. Detachment canalso be done by splitting the microstructured fiber at the fused jointf.ex. by using a CO₂— or eximer laser, ion milling or as in case of FIG.10 by wet or dry etching.

Microstructured Fiber

The microstructured fiber, which is used in making the double-clad fiberdevices, consists of three elements: the signal fiber(s) 14 with anactive core-element(s) 13, the capillary tube 11 having an air hole 12and one or more pump elements 10 shown in FIG. 5. These elements arefused together so that a structure, wherein the signal fiber(s) and pumpfiber(s) together form a solid all-fused inner cladding structure wherethe pump light propagates freely between the signal and pump fiber andprovides pump light for the active core element.

Capillary Tube as Bridging Element

The idea behind the capillary is that the capillary part 11 can beetched away from the fiber ends by feeding for example hydrofluoric acid(HF, room temperature) or sulphuric hexa-fluoride (SF₆, elevatedtemperature, 800° C.) into the air hole 12.

The etching process using HF is the following:SiO₂+4HF→SiF₄+2H₂O

When using 45% HF solution and assuming that 70% of the hydrofluoricacid is consumed the capillary diameter increases about 6%. The HF acidneeds to be chanced 12 times in the capillary if the outer diameter istwice the inner diameter. The larger hole diameter decrease the numberof the HF changes in the capillary. The etching of the capillary can bedone before coating removal and all the glass between the pump and thecore-elements can be removed.

This etching will detach the signal fiber 14 with the active core 13from the pump fiber(s) 10 as shown in FIG. 11. After the etching hasbeen done the free fiber ends 13, 14 can be spliced to fiber coupledpump diodes and to output (and input) signal fiber(s) using the standardsplicing techniques. After splicing the exposed fiber parts will bere-coated using the same polymers as were used for fiber coating.

Alternative method to partly separate the fiber elements from thestructure is to use laser micromachining methods either to the drawnfiber or to the preform prior to the drawing of the fiber. In the lattercase periodic holes are produced directly to the capillary part of thepreform. The separation distance in between the holes is determined bythe dimensions of the preform and by the desired absorption length inthe final fiber. For example for a 10-cm long preform with a fibergeometry shown in FIG. 5 and with a target core size of 30 μm theseparation of the holes in the preform is in the range of 1 to 2 mm. Thewidth of the hole is determined by the length of the fiber, which isneeded for splicing purposes. Typically this length in the final fiberis 50 cm, which corresponds to about 200 μm long hole in the preform.After the processing of these holes to the preform the fiber is drawnand coated. The double-clad devices can then be made simply by cleavingthe double-clad fiber in between the separation points, removing thecoating and splicing the signal and pump fibers from the composite fiberend to the signal and pump fibers.

Apart from acting as a separation element capillary tubes or axiallyextending air-holes in the structure can also bring additionalfunctionality in the double-clad fibers. The glass bridge surroundingthe air-hole and fusing all the elements together ensures efficient modemixing of the pump light in the structure as shown (see the non-circularsymmetry in FIGS. 5 and 6). This reduces the number of helical modes inthe double-clad fiber and therefore enhances the amount of pump lightabsorbed by the doped core. Further benefit of the air holes and theflexibility of the geometry within the microstructured fibers is that byarranging the air holes in close proximity to the doped core or byfilling the air holes with stress inducing material the birefringence ofthe fiber can be increased. This polarization maintaining effect can beincreased by arranging all the elements in a linear structure whichforces the fiber to bend only in two directions as shown in FIG. 7.Air-holes may also be filled with materials which enhance thenonlinearity of the fiber (e.g. poled fiber, Raman enhanced fiber).

Solid Glass Bridging Elements

The microstructured fiber may also be joined together via all-glassbridging elements. In this case a special joining structure ismanufactured using standard glass grinding methods as shown in FIG. 4 b.This glass joint can be partly separated from the structure by usinglaser micromachining methods, ion milling or laser-assisted wet etchingmethods either to the drawn fiber or to the preform prior to the drawingof the fiber. In the latter case periodic holes are produced directly tothe glass joint in between the rods in the preform. The separationdistance in between the holes is determined by the dimensions of thepreform and by the desired absorption length in the final fiber. F.exfor a 10-cm long preform with a fiber geometry shown in FIGS. 8 and 9and with a target core size of 30 μm the separation of the holes in thepreform is in the range of 1 to 2 mm. The width of the hole isdetermined by the length of the fiber, which is needed for splicingpurposes. Typically this length in the final fiber is 50 cm, whichcorresponds to about 200 μm long hole in the preform. After theprocessing of these holes to the preform the fiber is drawn and coated.The double-clad devices can then be made simply by cleaving thedouble-clad fiber in between the separation points, removing the coatingand splicing the signal and pump fibers from the composite fiber end tothe signal and pump fibers.

Sacrificial Cladding Layer as a Bridging Element

The microstructured fiber may also be joined together by a sacrificialcladding layer on an optical fiber acting as a bridging element in thebundle as shown in FIG. 10. In this case the joining of the structuresis done by the method shown in FIG. 4 c). The optical fiber 14 issurrounded by a sacrificial cladding layer 19 acting as a bridgingelement. Preferably, this cladding material is a silica glass doped witha material increasing the wet etch ratio such as phosphorous. On thejoining part some grooves are sawn on the surfaces of these roundobjects along their length. The grooves on the bridging elementpreferably do not extend over the sacrificial cladding layer. Pumpelements 10 are attached to the bridging element in such a manner thatthere are slits formed in the coating material between the surroundingfibers through which the sacrificial layer acting as bridging elementcan be worked externally. The detachment can be carried out by at leastpartly working and/or removing the corresponding length portion ofsacrificial layer of the optical fiber acting as a bridging elementbetween the fibers. Typically, said portion of the bridging element hasbeen removed by a method selected from wet etching and dry etching.

How the Fiber is Produced

All the preform elements are first made using the common techniques andthey are in the form of symmetrically shaped (round, rectangular,hexagonal, octagonal) tubes or rods. Typical shape of the rods and tubesis round. On the joining part some grooves are sawn on the surfaces ofthese round objects along their length. The resulting cross-section isshown in FIG. 4. After that the said elements are fixed together so,that the ‘teeths’ of the preform 1 fit to the grooves of the preform 2.Capillary tubes (4 a), all-glass joints (4 b) and optical fibers can beutilized as bridging elements. After that the preforms are fusedtogether (with flame or with oven) and the microstructured fiber isdrawn.

EXAMPLES Example 1

In one embodiment of the invention, the Key Hole fiber structure (cloverfiber) is established, FIG. 5. The fiber structure consists of Pumpfiber 10, capillary tube 11 with an air hole 12 in the middle of it,signal fiber 14 with an active core 13, low index polymer coating 15.

Example 2

In a second example, a Key Hole structure type 2 is created, FIG. 6. Thefiber structure consists of Pump fiber 10, capillary tube 11 with an airhole 12 in the middle of it, signal fiber 14 with an active core 13, lowindex polymer coating 15.

Example 3 Fiber with Forced Bending Direction

The compound fiber of the present invention allows for a number ofdifferent configurations as far as the size, shape, number and positionsof the each of its elements are concerned. Some of these configurationscan be used to enhance the properties of the fiber lasers or amplifiersbuilt from such fibers. For instance, it is evident that the fibergeometry of FIG. 7 makes the bending of the fiber to be very difficultalong one direction and very easy along the perpendicular direction.This property can be utilized in situations where the direction of fiberbending is important for controlling the modal properties of the activefiber. Thus, the active fiber geometry and orientation with respect tothe other elements can be fixed already in the preform manufacturingstage and the spatial relationships will remain the same during andafter the fiber drawing process. This property of the fiber of thepresent invention is of utmost importance especially in the case ofmulti-mode large mode area active fiber geometries and index profileswhere modal resonances or modal leakage out of the core can becontrolled by appropriate amount and direction of bending of the fiber.The Key Hole fiber structure 3 consisting of pump fiber 10, capillarytube 11 with an air hole 12 in the middle of it, signal fiber 14 with anactive core 13, low index polymer coating 15 is described in FIG. 7.

Example 4

In a further embodiment of this invention, the solid key fiber structure1 is established. The fiber structure is described in FIG. 8 consistingof pump fibers 10, signal fiber 14 with an active core 13, low indexpolymer coating 15.

Example 5

This embodiment of the invention encloses the solid key structure 2. Thefiber structure is described in FIG. 9 consisting of pump fibers 10,signal fiber 14 with an active core 13, low index polymer coating 15.

Example 6

This embodiment of the invention encloses the clover fiber structureutilizing a sacrificial cladding layer surrounding an optical fiber as abridging element. The fiber structure is described in FIG. 10 consistingof pump fibers 10, optical fiber 14 with an active core 13, low indexpolymer coating 15 and a sacrificial cladding layer surrounding theoptical fiber 14 acting as a bridging element 19.

Example 7

This embodiment of the invention describes the Key Hole fiber structurein a fiber laser/fiber amplifier configuration, after the capillary tube11 is removed from the structure and signal fiber 14 with an active core13 and pump fibers 10 are available for splicing. Fibers are fusedtogether along the length 16. The fiber structure in question isdescribed in FIG. 11.

Example 8

This embodiment of the invention describes the key fiber structure 1 ina fiber laser/fiber amplifier configuration, after the coating isremoved surrounding the pump fibers 10 and the signal fiber 14 with anactive core 13 at the location where the fibers are split apart fromeach other. After the separation the signal fiber 14 and pump fibers 10are available for splicing. Fibers are fused together along the length17. The fiber structure in question is described in FIG. 12.

Example 9

In a still another embodiment of the invention, the Key Hole fiberstructure 1 forms a pump coupler configuration, after the capillary tube11 is removed and the fused fiber bundle is heated and stretched. Theloose pump fibers 10 and the tapered output end of the fused bundle areavailable for splicing to fiber coupled pump diodes and to the doubleclad fiber. Fibers are fused together along the length 16. The fiberstructure in question is described in FIG. 13.

Example 10

The Key Hole fiber structure 1 can be established in a fiber laser/fiberamplifier configuration, after the capillary tube 11 is removed from thestructure and signal fiber 14 with an active core 13 and pump fibers 10are available for splicing. Fibers are fused together along the length16. The fiber structure in question is described in FIG. 14.

1. A process for preparing an optical fiber joint, in which process anoptical fiber or its undrawn precursor is brought into contact with alongitudinal element and attached to it, wherein a first object selectedfrom said optical fiber or its precursor and said longitudinal elementis provided with a ridge and a second object selected from said opticalfiber or its precursor and said longitudinal element is provided with agroove, the ridge is fitted into the groove, the optical fiber or itsprecursor and the longitudinal element are attached to each other, and,in the case of the optical fiber precursor, the attached optical fiberprecursor and the longitudinal element are drawn, to give said opticalfiber joint.
 2. A process according to claim 1, wherein said firstobject is provided with several parallel alternating ridges and groovesand said second object is provided with several parallel alternatinggrooves and ridges, whereby the ridges and grooves of the first objectare engaged with the corresponding grooves and ridges of the secondobject.
 3. A process according to claim 2, wherein the parallelalternating ridges and grooves form two saw-like patterns which areengaged.
 4. A process according to claim 1, wherein the optical fiber orits precursor and the longitudinal element are attached to each other byfusing.
 5. A process according to claim 1, wherein said longitudinalelement is another optical fiber or its precursor.
 6. A processaccording to claim 1, wherein said longitudinal element is alongitudinal bridging element, which is attached to said optical fiberand at least one other optical fiber to form a bundle of optical fibers.7. A process according to claim 6, wherein the bridging element isworkable and/or removable in order to detach the at least two fibersfrom each other.
 8. A process according to claim 7, wherein the fibersare attached to each other exclusively by means of the bridging element.9. A process according to claim 8, wherein the fibers are attached at adistance from each other which exposes the bridging element to working.10. A process according to claim 6, wherein the bridging elementconsists of a material, which has essentially the same refractive indexas the material of the outer layer of the fibers.
 11. A processaccording to claim 10, wherein the material of the bridging element isthe same as the material of the outer layers of the fibers and is alight transmitting material such as silica glass.
 12. A processaccording to claim 10, wherein the material of the bridging element ismore easily workable and/or removable than the material of the opticalfibers and preferably comprises silica glass which is doped to make itworkable/removable.
 13. A process according to claim 6, wherein at leastone of the fibers is attached to the bridging element by fusing.
 14. Aprocess according to claim 6, wherein the bridging element is a separateelement to which the at least two fibers are attached.
 15. A processaccording to claim 14, wherein the bridging element is a longitudinalelement having a cavity or hole.
 16. A process according to claim 15,wherein the longitudinal element is a capillary, which can be subjectedto internal working through said capillary.
 17. A process according toclaim 15, wherein the wall of the longitudinal element having a cavityor hole has a smaller cross-sectional area than the fibers.
 18. Aprocess according to claim 15, wherein the cavity or hole is filled witha functional substance such as an optical stress inducing material. 19.A process according to claim 6, wherein the bridging element is aprojection protruding from one of the at least two fibers and beingattached to the other of the at least two fibers.
 20. A processaccording to claim 1, wherein the bridging element is a cladding layeron one of the optical fibers.
 21. A process according to claim 20,wherein the cladding of the optical fiber is a sacrificial claddinglayer which is workable and or removable in order to detach the fibersfrom each other.
 22. A process according to claim 20, wherein the fibersare attached to each other exclusively via the sacrificial layer of theoptical fiber.
 23. A process according to claim 20, wherein thesacrificial layer consists of material, which has essentially the samerefractive index as the material of outer layer of the fibers is lighttransmitting such as doped silica glass.
 24. A process according toclaim 20, wherein the optical fiber is either a single cladding fiberwith a core of higher refractive index than the said light transmittingmaterial and a cladding of the said light transmitting material, or amultimode fiber consisting essentially of the said light transmittingmaterial.
 25. A process according to claim 11, wherein the bundlecomprises an outer cladding surrounding the fibers and bridging elementand consisting of a material having a lower refractive index than saidlight transmitting material.
 26. A process according to claim 6, whereinthe fibers and the bridging element are arranged in a geometry allowingbending only in a limited number of directions defined by the geometry.27. A process according to claim 11, wherein the fibers are selectedfrom single cladding fibers with a core of higher refractive index thansaid light transmitting material and a cladding of said lighttransmitting material, and multimode fibers consisting essentially ofsaid light transmitting material.
 28. A process according to claim 27,wherein the single cladding fibers are signal fibers or precursorsthereof and the multimode fibers are pump fibers or precursors thereof.29. A process according to claim 27, wherein at least one of the singlecladding fibers and/or at least one of the multimode fibers areadjacently attached to each other via the bridging element.
 30. Aprocess according to claim 29, wherein the single cladding fiber, thebridging element and the multimode fiber are, in said order, arranged ina parallel-aligned conformation and surrounded by the outer cladding.31. A process according to claim 30, wherein the single cladding fiber,the bridging element and the multimode fiber have differentcross-sectional areas.
 32. A process according to claim 31, wherein themultimode fiber has a larger cross-sectional area than the singlecladding fiber and the bridging element.
 33. A process according toclaim 29, wherein one of the single cladding fibers is parallel-alignedwith, in outwards order, two bridging elements and two of the multimodefibers, and surrounded by the outer cladding.
 34. A process according toclaim 29, wherein one bridging element is attached to three of themultimode fibers and one of the single cladding fibers and surrounded bythe outer cladding.
 35. A process according to claim 34, wherein thethree multimode fibers and the one single cladding fiber are arrangedaround the bridging element in an arrangement having a fourclover-formed cross-section.
 36. A process according to claim 6, whereina section of the bundle consists of said at least one optical fiber,which is detached from the bridging element.
 37. A process according toclaim 36, wherein the detachment has been carried out by at least partlyworking and/or removing a corresponding length portion of the bridgingelement between the fibers.
 38. A process according to claim 37, whereinthe portion of the bridging element has been removed by a methodselected from wet etching, dry etching, UV and ultra fast lasermicromachining as well as ion milling or a combination of said methods.39. A process according to claim 36, wherein the bundle sectionconstitutes an end section of detached fibers.
 40. A process accordingto claim 39, wherein the detached fibers of the end section areindividually covered with essentially the same material as that of saidouter cladding.
 41. A process according to claim 36, wherein at leastone further bundle portion thereof is tapered.
 42. A process accordingto claim 41, wherein the tapered bundle portion constitutes a taperedend portion.
 43. A process according to claim 36, wherein a furtherbundle section thereof consists of said at least two of the opticalfibers, which are detached from each other.
 44. A process according toclaim 43, wherein said bundle section is an intermediate section and atleast one of the optical fibers has been cut to establish opticalconnection.
 45. A process according to claim 43, wherein the furtherbundle section is the other end section of the bundle.
 46. An opticalfiber structure or precursor, comprising: an optical fiber or aprecursor of an optical fiber; and a longitudinal element, wherein oneof the optical fiber or the precursor of the optical fiber and thelongitudinal element is provided with a ridge, an other of the opticalfiber or the precursor of the optical fiber and the longitudinal elementis provided with a groove, the ridge being fitted into the groove suchthat the optical fiber or the precursor of the optical fiber and thelongitudinal element are attached to each other.
 47. An opticalstructure according to claim 46, comprising an optical fiber joint. 48.An optical structure according to claim 46, comprising an optical fiberbundle.